JPS6243976B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for separating high-boiling by-products that accumulate in a reaction zone during the production of isoprene.

Previously, the present inventors investigated a method for synthesizing isoprene by reacting isobutene and/or tertiary butanol (sometimes referred to as C ₄ ) with formaldehyde in the liquid phase, and found that C ₄ and A formaldehyde source is continuously or intermittently supplied into an acidic aqueous solution together with water, and the produced isoprene and unreacted raw materials are distilled out together with water, and the ratio of the number of moles of _C4 supplied at that time to the number of moles of formaldehyde and It has been discovered that isoprene can be produced in good yield by keeping the reaction pressure within a specific range (Japanese Patent Application Laid-open No. 70623/1983). If the reaction is carried out for a long period of time using this method, high boiling point by-products will accumulate in the reaction zone. This high-boiling byproduct undergoes phase separation from the acidic aqueous solution, but the difference in specific gravity between it and the acidic aqueous solution is extremely small, so if it remains as it is, it can be separated by a liquid separation operation that takes advantage of the difference in specific gravity, such as decantation. That's difficult. Furthermore, since the high-boiling by-product has the property of solidifying at room temperature, it is extremely difficult to handle it after separation.

The present inventors searched for a method to advantageously separate the high-boiling point by-products, and found that the by-products produced by the reaction and distilled out were easy to separate from the acidic aqueous solution and had a sufficient difference in specific gravity. We have found that high-boiling byproducts can be well dissolved not only at room temperature but also at room temperature, and have completed the following method of the present invention using this as a solvent.

That is, according to the present invention, isobutene and/or tertiary butanol, a formaldehyde source, and water are continuously or intermittently fed into an acidic aqueous solution to remove produced isoprene, low-boiling byproducts, water, and unreacted raw materials. In a method for producing isoprene by reacting a mixture containing the mixture with distillation, when separating high-boiling byproducts that accumulate in the reaction zone, an organic layer of the distillate from the above reaction is added to the reaction mixture or a part thereof. The residue obtained by distilling off unreacted raw materials and isoprene, or some components thereof (hereinafter simply referred to as solvent), is added to separate the organic layer containing the high-boiling by-products and an acidic aqueous solution. By doing so, high-boiling byproducts that accumulate in the reaction zone can be easily separated from the acidic aqueous solution. According to the method of the present invention, stable operation can be performed for a long period of time without substantially changing the state of the acidic aqueous solution.

A portion of the solvent used in the method of the present invention is consumed, but since it is a by-product of the reaction, there is no problem even if it is consumed. Specifically, the solvent is obtained as a distillation residue after distilling an organic layer distilled from the reaction and distilling off unreacted raw materials and isoprene. Compounds included in this include 4-methyl-5,6-dihydro-2H-pyran, methyl isopropyl ketone, 2-methylbutanal,
2,6-dimethyl-2,5-heptadiene, 2.
6-dimethyl-1,5-heptadiene, 3-methyl-3-buten-1-ol, 2-methyl-3-
Examples include buten-2-ol, but other compounds with various functional groups having 4 to 15 carbon atoms are also included. This distillation residue can be added as it is as a solvent, but it is also possible to use the remaining components after some of the compounds have been separated.

There is no particular restriction on the amount of solvent added, but if this amount is too small, the difference in specific gravity with the acidic aqueous solution will be small, and the viscosity of the separated high-boiling by-product will be high, which is disadvantageous in terms of handling. Also, if too much solvent is added, isoprene and other active ingredients dissolved in the acidic aqueous solution will be extracted.
It becomes necessary to recover it. From these points, the solvent is usually
It is added at a rate of 20-500g.

Methods for separating high-boiling byproducts from acidic aqueous solutions include leaving the reaction solution still in a reactor, or extracting a portion of the reaction solution and sending it to a decanter, extraction tower, etc., where high-boiling byproducts are separated. There are ways to separate them. The location where the solvent is added is appropriately selected depending on these separation methods.

When carrying out the method of the present invention, it is preferable that the ratio of the number of moles of C ₄ supplied to the number of moles of formaldehyde source supplied converted to formaldehyde (hereinafter referred to as C ₄ /FA) is at least 3. . When C ₄ /FA is less than 3, the yield of isoprene decreases. From the viewpoint of reaction yield, C ₄ /
The larger the FA is, the better it is, and there is no upper limit in the strict sense of the word, but even if these values are increased unnecessarily, the effect of improving the yield of isoprene will be small, and the amount of heat used will increase, which is economically disadvantageous. Therefore, C ₄ /FA should generally not exceed 20. In the present invention, since C ₄ is used in excess of formaldehyde as described above, most of the C ₄ supplied to the acidic aqueous solution is distilled out unreacted.
This material can be reused after being separated from other components. Since unreacted C ₄ has a composition close to the equilibrium composition of isobutene and tertiary butanol under the reaction conditions, as long as unreacted C ₄ is recycled to the reaction, isobutene and tertiary butanol are used as starting materials.
Even if one of the tertiary butanols is added to the reaction solution, a mixture of isobutene and tertiary butanol will ultimately be used as the reaction raw material.

When adopting a reaction method in which isoprene and low-boiling components are distilled out of the reaction zone together with water while supplying C ₄ , FA source, and water in an acidic aqueous solution, the pressure inside the reaction system can be adjusted to remove them from the reaction zone. The ratio of each component to be evaporated to water can be regulated; when the pressure is high, the proportion of water to the total of components other than water in the distillate decreases, and when the pressure is low, the opposite phenomenon occurs. happens. In order to obtain isoprene in a good yield, the pressure within the reaction system (however, if a low-boiling compound that is inactive under the reaction conditions is supplied with the raw material, the pressure after subtracting its partial pressure) is preferably adjusted to the reaction temperature of the acidic aqueous solution. It is preferable that the vapor pressure is within the range of 1.1 to 2.5 times the vapor pressure. Note that the vapor pressure (hereinafter referred to as Pw) of an acidic aqueous solution at the reaction temperature is a physical constant uniquely determined by the type and concentration of the acidic substance contained in the acidic aqueous solution. When the pressure inside the reaction system exceeds 2.5 times Pw, the yield of isoprene decreases significantly. When the pressure in the reaction system is less than 1.1 times Pw, no significant decrease in isoprene is observed, but the conversion rate of FA decreases and the ratio of water to isoprene in the distillate increases. The amount of heat consumed in the reaction increases.

In the method of the present invention, it is possible to carry out the reaction while supplying in addition to the reaction raw materials into the acidic aqueous solution, if desired, a low-boiling compound that is inactive under the reaction conditions. The low boiling point compound that is inactive under the reaction conditions that can be used here is a compound that does not substantially change before and after the reaction, and specifically, n-propane, n-
Examples include hydrocarbons having 1 to 10 carbon atoms such as butane, n-hexane, and cyclohexane, and inert gases such as nitrogen.

The catalyst used in the method of the present invention is an inorganic acid,
Acidic substances such as organic acids and their salts, which are used in the reaction zone in the form of aqueous solutions. The acid strength of these acidic aqueous solutions varies depending on the type of acidic substance, reaction temperature, supply rate of formaldehyde source, etc., but is usually selected from a pH range of 0.5 to 2.5. The acidic substance is preferably one that is low volatile or nonvolatile under the reaction conditions, and specifically includes inorganic acids such as phosphoric acid, sulfuric acid, and boric acid;
Heteropolyacids such as silicotungstic acid and phosphotungstic acid, p-toluenesulfonic acid, benzenesulfonic acid, trifluoromethanesulfonic acid,
Organic acids such as oxalic acid, and acid salts such as sodium hydrogen sulfuric acid may be mentioned.

Formaldehyde sources used in the method of the present invention include formaldehyde aqueous solutions, formaldehyde gas, etc. In addition, trioxane, which decomposes under the reaction conditions to give formaldehyde,
Paraformaldehyde and the like can also be used. Methyral and other formals can also be used. Since water is supplied to the reactor and formaldehyde takes the form of an aqueous solution in the reaction zone, it is advantageous in terms of reaction operation to use an aqueous formaldehyde solution as the formaldehyde source.

The isobutene and tertiary butanol used in the process of the invention may contain other hydrocarbons, 3-methyl-1,3-butanediol, 3-methyl-2-
Buten-1-ol, 3-methyl-3-butene-
1-ol, 3-methyl-1-buten-3-ol, methyl isopropyl ketone, 2-methylbutanal, methyl tert-butyl formal, 4.4
-dimethyl-1,3-dioxane, 4-methyl-
5,6-dihydro-2H-pyran and the like may be included. It is also possible to use alkyl tertiary butyl ethers, such as methyl tertiary butyl ether, which gives isobutene and tertiary butanol under the reaction conditions.

The reaction temperature suitable for the method of the present invention is determined by considering the acid strength of the acidic aqueous solution, and is usually 150 to 220°C.
Selected from the range of °C. If the reaction temperature is less than 150°C, the yield of isoprene will decrease even if the concentration of the acidic aqueous solution is increased to maintain the reaction rate at a constant level. Even if the reaction temperature exceeds 220°C, the yield of isoprene does not decrease significantly, but the conversion rate of formaldehyde under conditions that give the optimum selectivity decreases. If reaction conditions are selected to increase the conversion rate of formaldehyde, the sequential reaction from isoprene will increase, resulting in a decrease in the selectivity of isoprene.

The preferred rate of supply of the formaldehyde source to the acidic aqueous solution is determined by taking into account the acid strength of the acidic aqueous solution, the reaction temperature, and the reaction pressure. In order to increase the supply rate of the formaldehyde source, it is necessary to increase the acid strength of the acidic aqueous solution or to increase the reaction temperature, and in this case, the problem of corrosion of the reactor arises.
Therefore, the supply rate of the formaldehyde source is
Usually, when the formaldehyde source is converted to formaldehyde, it is preferably 3 mol or less per hour per 1 kg of acidic aqueous solution. There is no strict lower limit for the supply rate of the formaldehyde source, but if the supply rate is unnecessarily reduced, the reactor will become larger and disadvantageous in terms of equipment. When converted, it is preferably 0.2 mol or more per hour per 1 kg of acidic aqueous solution.

Examples are shown below to specifically illustrate the present invention.

Example 1 (i) Collection of distillation by-products Raw material inlet, distillation pipe, distillate condenser, stirrer, thermometer, pressure gauge, catalyst water outlet, catalyst water circulation line, high boiling product separation decanter, Heat exchanger,
Internal volume equipped with a decanter-type distillate tank
A 120 Hastelloy G-3 reactor was used. The volume of the catalyst water circulation line and heat exchanger is 25
, the volume of the high boiler separation decanter is 20. A total of 120 kg of 3% phosphoric acid water was put into the reactor, circulation line, heat exchanger and high-boiling decanter, heated by the heat exchanger through the circulation line while stirring, and the reactor was heated under a pressure of 16.0 kg/ ^cm2. The internal temperature was 178°C. Isobutene was then fed to an isobutene evaporator at a rate of 31 Kg/hr for vaporization, and was then fed to the reactor through a water evaporator. Water was supplied to the water evaporator at a rate of 8.0 kg/hr, vaporized together with isobutene, and supplied to the reactor. Next, a 27.67% formaldehyde aqueous solution was supplied to the reactor at a rate of 7.5 Kg/hr to start the reaction. The gas distilled from the reactor is condensed in a condenser and sent to a decanter-type distillate receiving tank.
Separated into organic and aqueous phases. The separated organic phase was sent to a distillation column, and isobutene was distilled off by continuous distillation. The still liquid was cooled and drained into a drum. The reaction and recovery distillation of isobutene were carried out for 100 hours while keeping the temperature and pressure of the reactor constant. During this time, the acidic aqueous solution was added at a rate of 15/hr.
The mixture was circulated through an overflow-type high-boiling decanter maintained at 160°C, but no overflow of high-boiling substances was observed. During this period, 680 kg of bottom liquid from isobutene recovery distillation was collected. This product was further distilled to distill out isoprene and tertiary butanol, yielding 105 kg of distillation residue. The distillation residue was analyzed by gas chromatography and confirmed to contain the following components.

4-Methyl-5,6-dihydro 4H-pyran
24.0% Methyl isopropyl ketone 11.4% 2,6-dimethyl-2,5-heptadiene
5.7% 2,6-dimethyl-1,5-heptadiene
3.1% 2-Methyl-3-buten-2-ol 3.0% 3-Methyl-3-buten-1-ol 2.1% On the other hand, after the reaction was completed, the liquid in the high-boiling decanter was extracted and the tar-like high-boiling material 8.3 Got Kg.
When this material was cooled to room temperature, it solidified and lost fluidity.

(ii) 300g/hr of the distillation residue obtained in (i) above.
The reaction was carried out under the same conditions as in (i) above, except that the high boiling material was fed to the circulation line to the decanter at a rate of . An overflow of oil was observed from the high-boiling decanter 20 hours after the start of the reaction, and 27 kg of oil was obtained by 100 hours after the start of the reaction. This product did not solidify even when cooled to room temperature.

Example 2 50 kg of the distillation residue obtained in (i) of Example 1 was taken, and fractions up to a boiling point of 122°C were distilled off to obtain 33.5 kg of residue. This removed methyl isopropyl ketone, 2-methyl-3-buten-2-ol and most of 4-methyl-5,6-dihydro-4H-pyran. 150g of this stuff/
The reaction was carried out under the same conditions as in (i) of Example 1, except that the mixture was fed to the circulation line to the high-boiling decanter at a rate of hr. An overflow of oily material was observed from the high-boiling material decanter 18 hours after the start of the reaction, and 18 kg of oily material was obtained by 100 hours after the start of the reaction. This product did not solidify even when cooled to room temperature.

Claims (1)

[Claims] 1. Continuously or intermittently feeding isobutene and/or tertiary butanol, a formaldehyde source, and water into an acidic aqueous solution while distilling a mixture containing formed isoprene, low-boiling byproducts, water, and unreacted raw materials. When separating high-boiling byproducts that accumulate in the reaction zone in a method for producing isoprene by reaction, unreacted raw materials and isoprene are added to the reaction mixture or a part thereof from the organic layer of the distillate from the reaction. A method for separating high-boiling by-products, which comprises adding a residue obtained by distillation or a part thereof, and separating the organic layer containing the high-boiling by-product and an acidic aqueous solution.